Ozone, as a powerful and clean oxidizing agent or germicide, is coming to attract attention. The use of ozone particularly for water treatment is increasing since treatment with ozone is advantageous. For example, since the product of ozone decomposition is oxygen, ozone-treated water is not caused to contain any residual substance (unlike the conventional chlorine-treated water). Further, the decomposition rate of ozone is so high that ozone itself does not remain in the treated water and, hence, there are no problems of secondary pollution.
For producing ozone which is a useful oxidizing agent as described above, electrical discharge methods and electrolytic methods have mainly been employed conventionally. The electrical discharge method has the advantages of a small energy unit requiring a low running cost. However, the disadvantages of this method are that the attainable ozone concentration is low, the ozone gas produced contains a large amount of impurities and the equipment tends to be large-scale. On the other hand, the electrolytic method has the disadvantage of a large energy unit, but has the advantage of being able to obtain a high-concentration of high-purity ozone gas through use of a small-sized electrolytic cell. At present, therefore, the electrolytic method is mostly employed because of the advantages of product purity and easiness of operations, although its energy unit is large.
In the electrolytic method, an electrolytic ozone generator (electrolytic ozonizer), employing as the anode a lead oxide-based electrode performing an excellent ozone-evolving function, is used; and, due to such an ozonizer, ozone can be obtained at a high concentration by conducting almost the same operation as that in ordinary water electrolysis. In this electrolytic ozone generator, pure water (ion-exchanged water) is used as the raw material and a perfluorocarbon sulfonic acid-based ion-exchange membrane is mainly used as a solid electrolyte, in combination with lead dioxide as the anode, to generate ozone. By this method, oxygen containing ozone gas at a concentration of about 15% can be obtained. The thus-produced ozone-containing oxygen may be used as it is or after being dissolved in water to give ozone-containing water. Since the ozone generator used in this electrolytic method has a simple structure and its operation is easy as described above, attention is now focused on this apparatus, which among small-sized ozonizers producing small amounts of ozone, is regarded as an ozone generator that generates ozone at a high concentration.
However, this ozone generator, although exhibiting excellent performance, has some drawbacks. The electrolytic cell of this apparatus normally employs a cation-exchange membrane as a solid electrolyte on which an anode and a cathode are tightly disposed, and pure water or ion-exchanged water is fed as raw water to the anode chamber side where ozone is evolved. In this apparatus, ozone and oxygen are evolved in the anode chamber and hydrogen ions, and other cations migrate, together with accompanying water, through the solid electrolyte to the cathode chamber, with hydrogen being evolved in the cathode. In this apparatus, all the cations generated in the anode chamber migrate through the solid electrolyte to the cathode chamber, whereas anions accumulate in the anode chamber because they are unable to pass through the solid electrolyte. Due to the accumulation of anions, the electrical conductance of the anolyte in the anode chamber increases and the current efficiency decreases.
Although the accumulated anions may be removed from the anolyte by circulating the anolyte to the outside of the cell and treating it, for example, with an ion-exchange resin, this treating method is difficult to adopt because the ion-exchange resin used is deteriorated by ozone contained in the anolyte being treated. For this reason, anolyte blowing is the only practical way to remove the accumulated anions. However, the anolyte blowing is disadvantageous in that the apparatus necessarily has an intricate structure. The anolyte blown off should be subjected to ozone decomposition treatment, etc., and the amount of the pure water or ion-exchanged water fed as raw water should be increased.